structure and type of sensila and their distribution in insect

insects have several sense organs to detect stimuli from environment and provide response against the stimuli

1. STRUCTRE OF MAJOR TYPES OF SENSILA AND THEIR DISTRIBUTION IN INSECT BODY SUSHMITA SHARMA Department of entomology Agriculture and Forestry University Rampur chitwan

2. INTRODUCTION Sensilla are the sensory organ of insects arising from cuticle of exoskeleton or lying beneath or within it Sensila are in the form of hair or pegs modified for sensation of different stimuli Each sensilum consists of two to three secondary neurons (Alexander, 2007) Sensila are abundant in antenna, palps and cerci though small quantity is found all over the body.

3. TYPES OF SENSILA MECHANORECEPTOR CHEMORECEPTOR AUDITORY RECEPTOR THERMOHYGRORECEPTOR PHOTORECEPTOR

4. MECHANORECEPTOR They are the sensory cells that detect the mechanical pressure due to movement of objects in the environment They are functionally divided into exteroreceptors ( mechanical forces) and proprioceptors ( position and orientation of body) (John C Tuthil , 2017) Exteroreceptors include Trichoform sensila, Campaniform sensila and stretch receptors Proprioreceptor contains crodotonal organ.

5. Tricoform sensila These cells are located in antennae and mouth parts These are hair like sense organ associated with spur and seta It consists of trichogen cell, tormogen cell, thecogen cell, glial cell as accessory supporting cell

6. Campaniform sensila These are dome shaped depression at the center of cuticle These are cap like structure made up of highly ductile material (Krishnan, 2015) They are found in wing base, halters, cerci, palp, base of trochanter, femur, tibia and tarsus

7. Stretch receptor Ephemeroptera, Plecopteran, Dictyopteran, Coleoptera and Dermaptera have a pair of stretch receptors on dorsal side of abdominal region Each receptor is connected with sensory neuron and one member of each pair has longitudinal and another has vertical orientation (M P, 1962) Multipolar stretch receptors are found near the joints of legs of cockroach which respond to low frequency stimulus (Guthrie, 1967)

8. Crodotonal organ Complex type of mechano receptor which consists of Johnston’s organ and subgenual organ Johnstons organ contains scolopidia arranged in bowl shaped structure Subgenual organ are located in proximal tibia of each leg Johnstons organ is located in antennae

9. AUDITORYRECEPTOR Auditory receptors consists of delicate tactile hair and tympanal ear. Tactile hair are found in plumose antennae of mosquito. Tympanal ear consists of tympanum, tracheal expansion and auditory crodotonal organ.

10. CHEMORECEPTOR These are the senses in insects which detect and response towards chemical substances in environment Chemoreceptor are divided into gustatory receptor and olfactory receptor Olfactory receptors for detection of chemicals in air through smell gustatory receptors for detection of dissolved chemicals through taste In aquatic insects chemoreceptors are restricted to gustatory receptor only

11. Olfactory receptor Mainly antennae and maxillary palp are used to detect sensation of odor Majority of olfactory receptive neurons are found in antennae Few of neurons are found in tarsus

12. Gustatory receptor Gustatory receptors stimulate the feeding behavior of insect Gustatory hairs and gustatory pegs of proboscis control opening of lobes and sucking food respectively ( Thoma , 2017) Taste sensilla are present on legs, wings, labellum and female genitalia

13. THERMOHYGRORECEPTOR Thermo receptor is temperature sensor and hygro receptor are moisture sensor These are non-porous and arise in different forms may be hair like structure or cap like structure In bugs thermohygroreceptor is used to select the host by detecting the temperature of host These receptors are found in antennae and wings

14. PHOTORECEPTOR Photoreceptor includes compound eye, dorsal ocelli and lateral ocelli

15. Compound eye Compound eye is major visual organ of insect which contains thousands of ommatidia Found in almost all adults and immature stages of ametabolous and hemimetabolous insects Ommatidia are functional unit of vision and their number varies from species to species Ommatidia are contain cornea, lens and photoreceptor cell

16. Dorsal ocelli They are simple eye often called only ocelli found in all adults and larva of non-holometabolous insects They are never found in the insect lacking compound eyes They appear vas 3-4 convex swelling at dorsal part of head They have single corneal lens covering several rhabdom

17. Lateral ocelli They are also called stemmata which are solely visual organ of holometabolous larvae Number of stemmata varies from one to six and lies laterally on the head They help to detect form, color and movement of object

18. CONCLUSION Insects detect and response to various stimuli occurring in the environment Mechanoreceptor, chemoreceptor, photoreceptor, and thermohygroreceptor are major types of sense organ found in insect body Different organs of insects are responsible for detecting stimuli Insects have variety of receptors which when stimulated pass information through nerve signals to central nerve system